The invention relates to a protective coating which is used as an anti-corrosion coating on substrates which are prone to corrosion, in particular corroding metals and/or their alloys and other materials, in particular as a base coat for the application of further porous coating systems or as a top coat, and to a method for its production and to the use on a coated substrate to protect against corrosion, and in particular for use against microbially influenced corrosion (MIC).
Until now, the problem with MIC corrosion has been countered by using tin paints. These paints, however, have a negative influence on all aquatic life. As a result, the use of paints containing tin has been banned in Europe, Canada and Japan. Copper-containing paints are still in use, but will be banned because of their toxicity. As a result, there is a need for novel paint systems which stop microbially influenced corrosion or substantially reduce the rate of corrosion.
The invention also relates to a sol-gel based anti-MIC coating for protection against corrosion and to the method for its production. The corrosion of steel in marine and onshore environments leads annually to a great deal of damage to structural elements. Depending on the environment, corrosion may have a number of causes.
Specifically, corrosion can be brought about by aqueous media which come into contact with steel. Especially if high concentrations of salt are present at the same time, for example in seawater, corrosion is accelerated. Another influencing factor which results in corrosion is due to the presence of what is known as “MICs” (microbially influenced corrosion). MICs are microorganisms which greatly accelerate corrosion, in particular in steels and ferrous alloys.
In order to characterize and classify the properties of an anti-corrosion coating, the corrosion resistance of the coating may be considered to be the criterion. This is typically determined using electrochemical impedance spectroscopy (EIS).
US 2010/0010119 A1 describes a coating with a curable epoxy resin mixture. The coating contains phyllosilicates functionalized with amino groups. These act as fillers. In that patent, the potential for segregation arises, which gives rise to different corrosion resistances. The corrosion resistance after 30 days is 109 Ωcm2.
US 2010/0119837 A1 discloses an anti-corrosion coating consisting of an epoxy-functionalized sol-gel network. In that patent, the epoxy acts as a cross-linking agent. Cross-linking is carried out with aromatic diamines. The coat-forming agents are tetraethoxyorthosilicate (TEOS) and tetramethoxyorthosilicate (TMOS). The epoxy used is 3-glycidoxypropyltrimethoxysilane (GPTMS). The cross-linking agent used is primarily phenyldiamine as a mixture of ortho-, meta- and para. The ratio of GPTMS to TMOS is 3:1, for example. The corrosion resistance of the coating is approximately 106 Ωcm2.
WO 2009/030959 A1 discloses a biologically functional sol-gel system which can be used for reducing/preventing biocorrosion. The system is composed of the components TEOS, methyltriethoxysilane (MTEOS) and GPTMS. The solvent used during production is ethanol. This coating solution may be supplemented with a bacterial suspension after conditioning. In order to ensure the vitality of the integrated microorganisms, it has to be supplied with nutrients. This is provided by means of the porosity of the coating. In order to improve the mechanical stability of the coatings, aluminium oxide particles were added. The resistance of the coating was determined using EIS and was 104 Ωcm2. One disadvantage of that invention is that the microorganisms are completely surrounded by the gel and thus cannot multiply within the coating. Since microorganisms only have a limited lifetime, the special action of this coating is coupled to the lifetime of the microorganisms in the coating. A further disadvantage is that the coating as a whole is porous due to its method of production. Thus, water can diffuse through the coating and reach the metal, which once more results in (conventional) corrosion of the metal substrate. Furthermore, alcohols are used as the solvent.
The coating in [1] is composed of TEOS and GPTMS. Thiourea is used as the cross-linking agent. The resistance of the coating is 105 Ωcm2.
An epoxy resin is used in [2]. It is modified with functionalized SiO2 particles. The hybrid material has a corrosion resistance of 105 Ωcm2.
[3] uses a purely organic epoxy resin coating. The electrochemical impedance measurements carried out on the coatings show that the maximum corrosion resistance of the coating is 107 Ωcm2.
[4] presents organic epoxy resins. These are based on bisphenol A. No further additives are used. The maximum corrosion resistance is 107 Ωcm2. The corrosion resistance dropped to 106 Ωcm2 after immersion for 7 days.
[5] presents a paint based on a vinyl chloride-vinyl acetate copolymer and an epoxy resin. Both variations have a corrosion resistance of approximately 108 Ωcm2.
In [6], a hybrid material is presented which consists of an epoxy resin and an organically modified silicate. In order to enhance the action as an anti-corrosion agent, inhibitors are integrated into the coating. N-(2-aminoethyl)-3-(trimethoxysilyl)propylamine is used as the silane precursor and supplemented with Araldite GY 257 (epoxy resin) after the former has been hydrolysed. With that coating system, corrosion resistances of less than 107 Ωcm2 were obtained. Immersion in 3.5% NaCl reduced the corrosion resistance to approximately 106 Ωcm2 after 12 days.
[7] discloses a hybrid material consist of an acrylate-derived polymer cross-linked with an epoxy-modified silane (GPTMS). TEOS is used as an inorganic coat-forming agent. Depending on the percentage composition of the organic and inorganic components, values of between 105 and 108 Ωcm2 are measured.
The coating described in [8] is composed of 2-mercaptobenzothiazole, polyaniline, polypyrrole, N-(2-aminoethyl)-3-(trimethoxysilyl)propylamine, diaminodiethylamine as well as an epoxy resin (Araldite GY 257). The maximum resistance obtained is below 106 Ωcm2.
In [9], zirconium dioxide is used as the coating agent. The coating solution produced and the type of coating technique employed are analogous to that of sol-gel chemistry. The coatings produced in that manner initially have a corrosion resistance of more than 1010 Ωcm2, i.e. they have very good anti-corrosive properties. However, on immersion in a 3.5% NaCl solution, in less than 7 days the resistance drops substantially to 108 Ωcm2.
The coatings in the references mentioned above in part have complicated compositions, in part are porous, and in part contain corrosion inhibitors, and as such are expensive. Solvents are necessary in order to apply the coatings. Purely organic coatings have a comparatively low thermal stability.
Thus, one aim of the present invention is to develop a simple protective coating based on readily accessible and therefore inexpensive starting materials, which coating has a high strength and has a high density, with very good anti-corrosion properties without the use of corrosion inhibitors and (organic) solvents.
The prior art reports the use of microorganisms which, when they metabolize, form components which can be used to prevent MIC.
WO 2010/095146 A1 contains a sol-gel based system which has an anti-corrosive action. In addition to various alkoxysilanes, an inhibitor is also used. In order to synthesize the materials for the coating, alcohols are used as the solvent. The disadvantage is that the use of additional inhibitors makes the system complicated and is associated with costs.
In WO 2011/000339 A2, aluminium oxide particles and epoxy compounds are used as the coating material. In order to prevent bacteria-induced corrosion of steel, a biocide based on pyrithione is used. Pyrithiones are harmful to health and thus cannot be used in open water or in environments. Escape of the biocide into the environment must be avoided at all costs.
In US 2013/0029134 A1, a sol-gel coating is described which essentially consists of an organic and an inorganic precursor. The sol which is produced is supplemented with polyaniline. Polyaniline acts as an anti-corrosive reagent. The disadvantage is that the composite system consists of more than one component, and so the thickness of the coating is difficult to set. Impedance measurements show a maximum resistance of approximately 107 Ωcm2. Within 10 days, this value dropped to 106 Ωcm2.
The further aim of the invention is to develop a simple anti-corrosion coating based on readily accessible and therefore inexpensive starting materials without using biocides or other types of harmful anti-MIC chemicals which on the one hand has a high density and, if appropriate, has porous regions with integrated organisms known as anti-MIC organisms which inhibit or kill corrosion-causing organisms (MICs).